1. Field of the Invention
The field of this invention is the inversion of inverse emulsion polymer products. This invention relates to the use of combinations of surfactants to improve the inversion of water-in-oil (w/o) inverse emulsion polymers.
2. Description of Related Art
An inverse, or water-in-oil, polymerization is utilized on a commercial scale to prepare high molecular weight water-compatible polymers. The water-in-oil emulsion polymer is termed in the scientific literature to be an inverse emulsion; this is based on its historical evolution as a variation of the oil-in-water (o/w) or latex emulsion technology, where water insoluble polymers are dispersed in an aqueous medium. Inverse emulsion refers to an aqueous (water) phase dispersed in a non-aqueous (oil) phase, where the aqueous phase and oil phase can also be referred to as the respective discontinuous and continuous phases. The resultant water-compatible polymers are in the discontinuous aqueous phase.
The advantages of this process include the ability to prepare high molecular weight, water-compatible polymer in a low viscosity, high solids form. This process produces emulsions that are 20-50% polymer, exhibit low viscosity, and the polymer can have a molecular weight in the tens of millions. In contrast, other methods for preparation of high molecular weight water-soluble polymers include solution polymerization of water-soluble monomers, which results in a low solids solution (<5% polymer), and/or high solution viscosity, and/or lower product molecular weight. The low product solids can also be more costly to ship.
Inverse emulsion polymers are prepared and stabilized using surface active agents, more commonly known as surfactants. The surfactants utilized will permit the emulsification of the water soluble monomer in the oil phase prior to polymerization, and provide stability to the resultant emulsion polymer. Stability, which includes resistance to settling, minimal changes in viscosity with time and premature inversion, not to mention the need for a stable emulsion during the polymerization process, requires a robust emulsion stabilization package. Emulsion surfactant systems have been developed that provide excellent emulsion stability.
Inversion of the emulsion refers to the process prior to use, where the phases are reversed, and the polymer is released from the discontinuous phase. Addition of a large volume of aqueous solution is a key aspect of the inversion process. This results in a continuous aqueous (water) phase where the coalescence of the previously dispersed aqueous phase results in the dispersal of the polymer in solution, resulting in a viscosification of the solution. Inversion is assisted by the addition of surfactants, termed ‘breaker surfactants’, to the emulsion that will help disrupt the original emulsion stabilization system when the relatively large volume of water is combined, using some level of agitation or shear, with the water-in-oil emulsion. It is the joint action of these three factors, the large volume of dispersed phase, the shear forces, and the breaker surfactant(s), that results in the inversion, or phase reversal, of the emulsion. Moreover, the polymer is now available to interact with other aqueous phase materials. The relative smaller amount of oil (20-40% by weight of the original emulsion) becomes dispersed in the water phase, where, due to the addition of the large volume of aqueous solution, the oil is a minor component.
The polymer is inverted into an aqueous solution, such that the resultant concentration of active polymer typically ranges from 0.1% to 1.0% by weight. The concentration utilized depends upon numerous factors, including but not limited to, the water chemistry and temperature, solution viscosity, feed rates, and equipment size and flow rates.
The emulsion polymer may be inverted into an aqueous solution utilizing either a batch, continuous, or semi-continuous system. In a batch process, the neat emulsion is fed into the agitator vortex of a vessel containing water until the target concentration is achieved. The polymer is then mixed until homogenous. In a continuous or semi-continuous system, convergent flows of water and neat emulsion at the desired concentrations are allowed to contact. The resultant mixture is then passed through a mixing stage, such as a static mixer or mechanical pump, where the mixing action enhances the inversion process. The aqueous solution is then typically transferred into a tank, where it is mixed until homogenous. In a continuous system the step of transferring to a tank is eliminated.
Additional dilution water is typically added to the inverted polymer solution just prior to introduction into the process to aid in dispersal of the polymer.
It is known that the use of an additional surfactant helps facilitate the inversion of the emulsion and the introduction into and subsequent viscosification of the now continuous water phase; this surfactant is often termed an ‘inverter’, ‘inverting surfactant’ or ‘breaker’ (the term ‘breaker’ will be used). To avoid confusion, the term ‘emulsification surfactant’ is used here to refer to surfactant(s) used to stabilize the emulsion. The term ‘breaker surfactant’ is used to refer to surfactant(s) used to break or invert the emulsion. The term ‘emulsion stabilization package or system’ refers to one or more emulsification surfactants. The term ‘breaker package or system’ refers to one or more breaker surfactants.
Breaker surfactant(s) are typically added after polymerization into the resultant emulsion polymer, but can be added as part of the emulsion stabilization package. These emulsions are called self-inverting surfactants, as they will self invert under the conditions of high shear in an excess of water. Alternatively, the breaker surfactant(s) can be added to the inverse emulsion at the application site, or can be added to the water prior to the addition of the inverse emulsion. The inversion step is critical to the utility of the emulsion polymer product, for it places the polymer in the continuous aqueous phase where it can then act as a flocculant, coagulant, dispersant, or rheological agent.
The commercial viability of water-in-oil emulsions is dependent on the ability to easily and effectively invert the emulsion. The time required to invert the emulsion is an important product attribute. The time period for a solution to reach its maximum viscosity is a good measure of inversion time, as this is an indication the polymer has fully equilibrated from the emulsion into the aqueous solution. Another indication of inversion time is the solution conductivity after polymer addition. A poorly inverting emulsion will exhibit low conductivity as the polymer remains within the discontinuous phase, whereas an emulsion with good inversion properties will exhibit a high conductivity after introduction into water, as the polymer has dispersed into the aqueous solution.
Inversion is typically done just prior to use at the application site. Key to this process is selection of the breaker surfactant system. The breaker surfactant system must have a chemical nature that will help disrupt the original emulsion stabilization system during the inversion process, facilitating reversal of the phases, and permit the polymer to be fully introduced into the aqueous solution. However, the other key requirement for a commercially viable emulsion product is that the emulsion must be stable during the period between manufacture and use. Thus, a balance must exist between product stability and ease of inversion.
Emulsion surfactant systems for use in inverse emulsion polymers have been developed that provide excellent emulsion stability; these systems, consequently, can be difficult to invert. Among the most difficult to invert stabilization systems are those that contain one or more diblock and triblock polymeric surfactants. Other inverse emulsion water-soluble polymer systems that are inherently difficult to invert are those where the polymer (or its monomers) contain hydrophobic groups or surface active groups.
Inverse emulsion polymerization is a standard chemical process for preparing high molecular weight water soluble polymers. Such processes are known to those skilled in the art, for example see U.S. Pat. No. 3,284,393 and Reissue U.S. Pat. Nos. 28,474 and 28,576. Use of breaker surfactants is known in the art see for example U.S. Pat. Nos. 3,624,019 and 3,734,873 which describe the inversion of inverse emulsion prepared by dissolving polymer powder into a water-in-oil emulsion, then affecting the inversion rate by the addition of an alkyl phenol ethoxylate to the emulsion or the water. U.S. Pat. No. 5,925,714 claims a self-inverting inverse emulsion where an alkoxylated castor oil is utilized at levels of 0.5% to 7.0% as the breaker surfactant. The invention is noted as an improvement in the polymer flocculant efficiency due to improved inversion.
It is known in the art to use polymeric surfactants to stabilize inverse emulsions. Polymeric emulsifying surfactant system may comprise one or more polymeric surfactants.
There remains a need in the field to find inversion systems that are more effective then those that are presently known.